A new technique permits spreading of DNA restriction fragments, and small DNA-protein complexes on uncoated, thin carbon foils for molecular weight measurement by Scanning Transmission Electron Microscopy. Applied to E. coli RNA polymerase, the method yields a molecular weight for the holoenzyme bound to a restriction fragment of phage T7 DNA of 453,000 + 8,000 (standard error of themean, n = 15), in agreement with recent gel determinations to within 1%. In a collaboration with James Manley of Columbia University, a restriction fragment of adenovirus 2 containing the major late promoter is incubated for transcription with a soluble extract of uninfected HeLa cells. Among the particles found bount to the DNA, one prominent group has a mean molecular weight of 202,000+4.0%, where the error quoted is the internal standard deviation for individual measurements and is in agreement with the predicted error. The M.W. measurement is evidence for nucleosome formation: a core nucleosome consisting of 8 core histones, no H1, and 145 bp of DNA, has a M.W. of 198,000. Particles of greater size and mass, potentially assemblages of RNA polymerase II subunits and transcription initiation factors, are observed bound near the Ad 2 major late promoter (manuscript in preparation). Via collaboration, the binding of biochemically purified RNA polymerase II, with and without additional factors, will be studied. Determination of the factors required for specific binding, and their geometrical relationships to Pol II, to the Ad 2 promoter (or DNA in general), and to each other, is of the greatest interest for understanding mechanisms of eukaryotic transcription. The SV40 template will be used to characterize via binding the three early promoter regions discovered by Chambon. The specifically bound particles found using the Manley extract will be interpreted in terms of the binding of the biochemically purified components. In the control study of the binding of E. coli RNA polymerase to T7 restriction fragments, omission of fixation leads to variable fragmentation of the enzyme; the smallest fragment bound at or near a promoter is found to have a M.W. equal to that of Sigma plus one of the large subunits (Beta or Beta'). Such an implication of Sigma + Beta or Sigma + Beta' in the specific binding of RNA polymerase to DNA is in agreement with the results of others. Similar fragmentation studies will be carried out for Pol II. Efforts to extend the M.W. measurements to polymerases which are elongating a transcript are in progress.